EU-A 87107089 discloses a twin shaft vacuum pump of this species. The rotors are each respectively equipped with a projection (claw tooth) and with a recess and execute their rotational motion in meshing and non-contacting fashion in the pump chamber. The respective recesses control the admission and discharge openings situated in the lateral shields of the pump chamber. During the synchronous motion of the rotors, pump volumes that are sealed by gap openings and initially enlarge and then again diminish are formed, these pump volumes compressing the gas in-flowing at the suction side and conveying it to the delivery side.